This invention relates generally to tri-level imaging and more particularly to a method and apparatus for reducing the amount of background toner particles deposited on a final substrate during the transfer of a tri-level image from a charge retentive surface to a substrate such as plain paper.
In the practice of conventional xerography, it is the general procedure to form electrostatic latent images on a xerographic surface by first uniformly charging a charge retentive surface such as a photoreceptor. Only the imaging area of the photoreceptor is uniformly charged. The image area does not extend across the entire width of the photoreceptor. Accordingly, the edges of the photoreceptor are not charged. The charged area is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images. The selective dissipation of the charge leaves a latent charge pattern on the imaging surface corresponding to the areas not exposed by radiation.
This charge pattern is made visible by developing it with toner by passing the photoreceptor past a single developer housing. The toner is generally a colored powder which adheres to the charge pattern by electrostatic attraction. The developed image is then fixed to the imaging surface or is transferred to a receiving substrate such as plain paper to which it is fixed by suitable fusing techniques.
In tri-level, highlight color imaging, unlike conventional xerography, the image area contains three voltage levels which correspond to two image areas and to a background voltage area. One of the image areas corresponds to non-discharged (i.e. charged) areas of the photoreceptor while the other image areas correspond to discharged areas of the photoreceptor.
The concept of tri-level, highlight color xerography is described in U.S. Pat. No. 4,078,929 issued in the name of Gundlach. The patent to Gundlach teaches the use of tri-level xerography as a means to achieve single-pass highlight color imaging. As disclosed therein the charge pattern is developed with toner particles of first and second colors. The toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged. In one embodiment, the toner particles are supplied by a developer which comprises a mixture of triboelectrically relatively positive and relatively negative carrier beads. The carrier beads support, respectively, the relatively negative and relatively positive toner particles. Such a developer is generally supplied to the charge pattern by cascading it across the imaging surface supporting the charge pattern. In another embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brushes. Each brush supplies a toner of one color and one charge. In yet another embodiment, the development systems are biased to about the background voltage. Such biasing results in a developed image of improved color sharpness.
In highlight color xerography as taught by Gundlach, the xerographic contrast on the charge retentive surface or photoreceptor is divided three, rather than two, ways as is the case in conventional xerography. The photoreceptor is charged, typically to 900 v. It is exposed imagewise, such that one image corresponding to charged image areas (which are subsequently developed by charged-area development, i.e. CAD) stays at the full photoreceptor potential (V.sub.cad or V.sub.ddp, shown in FIG. 1a). The other image is exposed to discharge the photoreceptor to its residual potential, i.e. V.sub.dad or V.sub.c (typically 100 v) which corresponds to discharged area images that are subsequently developed by discharged-area development (DAD) and the background areas exposed such as to reduce the photoreceptor potential to halfway between the V.sub.cad and V.sub.dad potentials, (typically 500 v) and is referred to as V.sub.white or V.sub.w. The CAD developer is typically biased about 100 v (V.sub.bb, shown in FIG. 1b) closer to V.sub.cad than V.sub.white (about 600 v), and the DAD developer system is biased about 100 v (V.sub.cb, shown in FIG. 1b) closer to V.sub.dad than V.sub.white (about 400 v).
After development of the tri-level image is complete, a pre-transfer step must be performed in order to make all of the toner on the photoreceptor (both colors) common in polarity so that conventional transfer methods can be utilized. For sake of illustration, it is assumed that a pre-transfer device is operating in the positive mode, and that transfer is operating negatively. When the developed tri-level image is exposed to a positive pre-transfer dicorotron, the negative charge on the color toner changes to positive, making it common in sign with the black toner. This enables transfer of the developed image to paper using negative corona. However, because the low charged and/or wrong sign toner present in the background areas is also exposed to the pre-transfer dicorotron, it also becomes positive (or more positive in the case of the wrong-sign color background). As a result, the background toner also tends to transfer to paper, which results in visible background on the fused tri-level prints.
It is well known in the prior art to subject a developed image on a charge retentive surface to corona discharge prior to image transfer for various reasons. For, example, U.S. Pat. No. 3,444,369 issued on May 13, 1969 relates to a method and apparatus for the reduction of background in transferred xerographic copy. A developed toner image on a photoconductive surface is subjected to a low level corona discharge of a polarity opposite the charge on the toner particles overlying the image areas. The corona discharge adjacent the image areas will be repelled by the like sign, but highly charged image areas of the photoconductive surface to thereby render the image area toner unaffected. The corona discharge adjacent the non-image areas of the photoconductive surface will not be repelled and will thus convert the toner overlying the non-image areas to a polarity opposite that on the image area toner particles. This will permit the electrostatic transfer of the image area toner, but will tend to suppress the transfer of the non-image area toner to a backing sheet.
It is also known to utilize light exposure and corona discharge prior to image transfer as shown in U.S. Pat. No. 4,506,971. In this device the light exposure occurs prior to the corona exposure. As stated therein, blurred images are minimized or eliminated in a xerographic reproduction prior to transfer by first exposing the image to light to at least substantially discharge the background around the image and to reduce the charge on the photoreceptor holding the image thereto. Secondly, a charge of opposite polarity of the charged photoreceptor is deposited onto the image and photoreceptor. This, as stated, produces a very stable image for transfer since a very strong holding force is produced to hold the image in place as the image enters the transfer station.
U.S. Pat. No. 3,784,300 issued on Jan. 8, 1974 relates to a copying apparatus with a pre-transfer station including a pre-transfer corotron and lamp arranged such that the light exposure of the photoreceptor is subsequent and not simultaneous with the pre-transfer corona charging.
U.S. Pat. No. 4,205,322 issued on May 27, 1980 relates to an electrostatic recording apparatus in which a toner image consisting of toner particles of at least two different kinds and of different polarities is efficiently and reliably transferred to a recording medium such as an ordinary sheet of paper. The toner particles having different polarities are all converted into those having one polarity and after such conversion the toner image (with its two kinds of particles) is electrostatically transferred to the recording medium, the transfer involving both kinds of particles at the same time.
U.S. Pat. No. 5,038,177 granted to Parker et al on Aug. 6, 1991 describes that balanced, efficient corona transfer for both the charged area image and the discharged area image of a developed tri-level image is obtained by the provision of a selective pretransfer charge corona device in combination with a pretransfer discharge lamp. While improved transfer over prior art devices is obtained using a pretransfer lamp prior to pretransfer charging the preferred embodiment of the invention utilizes a pretransfer lamp before and in coincidence with pretransfer charging. In this patent the pretransfer lamp is positioned adjacent the side of the photoreceptor opposite the toner images for controlling the magnitude and distribution of pre-transfer current so that disproportionately more charge is added to the part of composite tri-level image that must have its polarity reversed compared to elsewhere on the image.
U.S. patent application Ser. No. 08/179,176 filed in the name of Pietrowski et al on Jan. 10, 1994 discloses pre pretransfer treatment for multiple toner images for increasing the operating latitude for pretransfer/transfer. In one embodiment of the invention, a pre pretransfer corona device is used to drive the tribos of two multiple toner images toward each other prior to pretransfer. A single constant current corona discharge device is used in that embodiment. Subsequent pretransfer treatment serves to reduce the delta tribo between the two images thereby providing an operating latitude of 3 micro coul/g.